Sound is defined as a mechanical compression and rarefaction or a longitudinal displacement wave that propagates through a medium (solid, liquid or gas). The speed of this propagation depends on the type and temperature of the medium. It does not depend the pressure of the medium. In dry air at 20° C (68° F) the speed of sound is approximately 343 m/s. (A real-world estimate sometimes used when precision isn't important is 1 meter per 3 milliseconds.) Sounds can often be thought of as having two components: frequency and amplitude. The frequency refers to the rate of change within the wave and is measured in hertz (Hz). The amplitude refers to the magnitude of pressure change within the wave. While the pressure can be measured in pascals, the amplitude is more often referred to as sound pressure level and measured in decibels, or dB SPL, sometimes written as dBspl, dB(SPL), or dBSPL. When the measurement is adjusted based on how the human ear perceives loudness based on frequency, it is called dBA or A-weighting. See decibels for a more thorough discussion. Sounds that are sine waves with a single pair of frequency and amplitude components are generally perceived as a pure tone. (While sound waves tend to be thought of as being sine waves, that is not always true. Square waves, sawtooth waves, triangle waves, and many other types are possible and they all have a distinct sound. For convenience in this article, however, it is best to think of sine waves.) Sounds are usually much more complex than that, consisting of multiple tone components and harmonics. While the sound may still be referred to in general use as a single frequency (for example, a piano striking the A above middle C is said to be playing a note at 440 Hz) the sound you perceive will be colored by all of the frequency components and their relative amplitudes (see timbre.)
The frequency range of sound audible to humans is approximately between 20 and 20,000 Hz. This range varies by individual and generally shrinks with age. It is also an uneven curve -- sounds near 3000 Hz are often perceived as louder than a sound with the same amplitude at a much lower or higher frequency. Above and below this range are ultrasound and infrasound, respectively. The amplitude range of sound for humans has a lower limit of 0 dB SPL (sometimes called the threshold of hearing). While there is technically no upper limit, sounds begin to do damage to ears at 85 dB SPL and sounds above approximately 130 dB SPL (called the threshold of pain) cause pain. Again, this range varies by individual and changes with age.
The perception of sound is the sense of hearing. In humans and many animals this is accomplished by the ears, but loud sounds and low frequency sounds can be perceived by other parts of the body through the sense of touch. Sounds are used in several ways, most notably for communication through speech or, for example, music. Sound perception can also be used for acquiring information about the surrounding environment in properties such as spatial characterics and presence of other animals or objects. For example, bats use one sort of echolocation, ships and submarines use sonar, and humans can determine spatial information by the way in which they perceive sounds.
The study of sound is called acoustics and is performed by acousticians. A notable subset is psychoacoustics, which combines acoustics and psychology to study how people react to sounds.